Try figure out which note it is. Say for example it's an A and the other is an E.
The gap is 5. Which makes it perfect. Therefore it is a perfect 5th. Here's a chart to help remember.

Perfect- 1,4,5,8
Major- 2,3,6,7

If it's perfect and made flat, depending on the key signature or if there is an accidental, then it becomes diminished. If it's made sharp, then it's augmented. If both notes are flat or sharp, then it goes back to either minor or perfect. If it's major and made flat, then it's minor, then if there are 2 flats it's diminished.

another exercise you can do is play a note and try to sing an interval above it.

train yourself on octaves/unisons, fifths, and fourths first (in that order). play an A, and sing an A. then play an A, and sing a different A, either an octave higher or lower.

once you're good with that, play an A and sing an E.

and once you can do that, play an A and sing a D.

once you have that, get thirds and sixths down. major third, minor third, major sixth, minor sixth. same method. the order for this doesn't really matter, but personally i found thirds to have been easier than sixths.

then seconds and sevenths. same deal. seconds are easy intervals to sing melodically, but at first they're a little difficult to sing with a harmonic interval a second away.

ultimately, you want to be singing EVERYTHING if you train you ear -- the fastest way to internalize pitch and music is to sing it.

another exercise you can do is play a note and try to sing an interval above it.

train yourself on octaves/unisons, fifths, and fourths first (in that order). play an A, and sing an A. then play an A, and sing a different A, either an octave higher or lower.

once you're good with that, play an A and sing an E.

and once you can do that, play an A and sing a D.

once you have that, get thirds and sixths down. major third, minor third, major sixth, minor sixth. same method. the order for this doesn't really matter, but personally i found thirds to have been easier than sixths.

then seconds and sevenths. same deal. seconds are easy intervals to sing melodically, but at first they're a little difficult to sing with a harmonic interval a second away.

ultimately, you want to be singing EVERYTHING if you train you ear -- the fastest way to internalize pitch and music is to sing it.